1. Field of the Invention
The present invention relates to communications systems, communications control apparatuses and methods, and computer programs therefor for enabling a plurality of wireless stations to intercommunicate with one another, and more particularly relates to a communications system, a communications control apparatus and method, and a computer program therefor for configuring a network under the control of a specific control station.
More particularly, the present invention relates to a communications system for enabling a plurality of wireless networks to coexist with one another and to a communications control apparatus and method and a computer program therefor for controlling a communications operation in each wireless network under a communications environment in which a plurality of wireless networks are in contention with one another. More specifically, the present invention relates to a communications system for enabling a plurality of wireless networks that are in contention with one another in the same frequency band to coexist with one another and to a communications control apparatus and method and a computer program therefor for controlling a communications operation in each communications network under a communications environment in which a plurality of wireless networks are in contention with one another in the same frequency band (“the same frequency band” includes the Ultra-Wideband (UWB) wireless communications for performing data transmission and reception by spreading the data over a very wide frequency band).
2. Description of the Related Art
A plurality of computers is connected with one another to configure a local area network (LAN) to share information such as files and data, to share peripheral devices such as printers, and to exchange information by transferring email and data content.
Known LANs are configured by establishing wired connections using optical fibers, coaxial cables, or twisted-pair cables. In this case, a circuit laying construction is necessary, which makes it difficult to configure the network. Also, the cable laying is complicated. After the LAN has been configured, the movable range of each apparatus is restricted by the cable length, which is inconvenient. A system that liberates the user from the wiring in such a known wired LAN is a wireless LAN, which has drawn public attention. According to this type of wireless LAN, most of the wiring or cables can be omitted in work space such as an office. A communications terminal such as a personal computer (PC) can thus be moved relatively easily.
Due to an increase in the speed and a decrease in the cost of the recent wireless LAN systems, the demand therefor has been tremendously growing. In particular, recently introduction of a personal area network (PAN) has been studied in order to perform information communications by configuring a small wireless network among a plurality of personal electronic apparatuses. For example, different wireless communications systems are defined using the frequency band, such as the 2.4 GHz band or the 2.5 GHz band, which does not require a license from the competent authorities.
For example, the IEEE (Institute of Electrical and Electronics Engineers) 802.15.3 Working Group has been conducting standardization activities of high-rate wireless personal area networks (WPANs) exceeding 20 Mbps. The corresponding section recommends the standardization in compliance with a physical (PHY) layer that mainly uses signals in the 2.4 GHz band.
In this type of wireless personal network, one wireless communications apparatus operates as a control station referred to as a “coordinator”, and a PAN is configured around the coordinator within a range of approximately 10 m. The coordinator cyclically transmits a beacon signal in a predetermined period. A period bounded by transmission of consecutive beacon signals is defined as a transmission frame period. In each transmission frame period, time slots to be used by wireless communications apparatuses are allocated.
As the time slot allocation method, for example, “guaranteed time slot” (GTS) and “dynamic time division multiple access (TDMA)” methods are adopted. Such communications methods dynamically allocate transmission bands while ensuring a predetermined transmission capacity.
For example, a contention access period (CAP) and a contention free period (CFP) are provided for a MAC (Media Access Control) layer to be standardized by IEEE 802.15.3. In the case of asynchronous communications, CFP is used to exchange short data or command information. In contrast, stream communications is performed by a mechanism involving dynamically allocating a GTS to perform channel-allocated transmission.
The MAC layer to be standardized by IEEE 802.15.3 is defined to accommodate standard specifications for PHY layers other than the PHY layer that uses signals in the 2.4 GHz band. Also, standardization activities for using, as the PHY layer to be standardized by IEEE 802.15.3, a PHY layer other than that using signals in the 2.4 GHz band have been gradually started.
Recently, wireless LAN systems using spread spectrum (SS) have been put into practice. In addition, UWB transmission using SS, which is targeting applications such as PAN, has been proposed.
In direct-sequence spread spectrum (DS-SS), which is one type of SS, an information signal at the transmitter side is multiplied by a random code sequence, which is referred to as a pseudo noise (PN) code, thereby spreading the information signal over a wider bandwidth, and the information signal is transmitted. At the receiver side, the received spectrum-spread information signal is multiplied by the PN code to de-spread and read the information signal. In UWB transmission, the spreading ratio applied to the information signal is increased to a maximum. High-rate data transmission is realized by performing transmission and reception by spreading data over, for example, a very wide frequency band of 2 GHz to 6 GHz.
UWB uses a signal sequence of extremely short duration (approximately 100 pico seconds) impulses to configure an information signal, and the signal sequence is transmitted/received. The occupied bandwidth is the band in the order of GHz, where the occupied bandwidth divided by its center frequency (for example, 1 GHz to 10 GHz) is approximately one. The occupied bandwidth is much wider than the bandwidth that is generally used by a wireless LAN using the so-called W-CDMA, cdma 2000, SS, or orthogonal frequency division multiplexing (OFDM).
FIG. 13 illustrates an example of data transmission using UWB. Input information 901 is spread by a spread code 902. Multiplication of the input information 901 by the spread code 902 may be omitted depending on the type of system using UWB.
Spectrum-spread information signal 903 is modulated using a UWB impulse signal (wavelet pulses) to generate a signal 905. The possible modulation schemes include pulse position modulation (PPM), biphase modulation, amplitude modulation (AM), and the like.
Since the UWB impulse signal consists of extremely narrow pulses, in terms of frequency spectrum, a very wide band is used. The power of the input information signal thus becomes less than or equal to the noise level in each frequency area.
Although the received signal 905 is lost in noise, the received signal 905 is detectable by computing a correlation value between the received signal 905 and the impulse signal. Since signals are spread in many systems, many impulse signals are transmitted with respect to one bit of transmitted information. A reception correlation value 907 of the impulse signal can be further integrated with respect to the length of the spread code 902 to generate an integrated signal 908. Accordingly, the transmitted signal is detected more easily.
The spread signal generated by the UWB transmission scheme only has a power less than or equal to the noise level in each frequency area. For this reason, a UWB-transmission-based communications system can coexist with other types of communications systems in a relatively easy manner.
A communications environment will now be considered that includes many apparatuses in the office due to the widespread use of information apparatuses such as PCs, the apparatuses being linked with one another by wireless networks. Two or more wireless networks may reside in the small work environment. In such a case, the plural wireless networks coexist with one another in the same frequency band. The “same frequency band” includes the UWB wireless communications for performing data transmission and reception by spreading the data over a very wide frequency band.
The specification for the PHY layer using signals in the 2.4 GHz band, which is to be standardized by the above-described IEEE 802.15.3, must take into consideration the coexistence with other wireless communications systems that operate in the same frequency band.
One known method,for enabling wireless networks to coexist with one another is a “Child Piconet” method described in the IEEE P802.15.3 Draft 0.9. The “Child Piconet” method allows a communications apparatus included in a network serving as a parent to generate a child network under the control of a control station for the parent network and to operate the child network. Specifically, a portion of a frame period used by the parent network is allocated as a frame period used by the child network.
Another method for enabling wireless networks to coexist with one another is a method for configuring a “Neighbor Piconet”, which is described in the IEEE P802.15.3 Draft 0.9. According to this method, control stations for two independent wireless networks each allocate a band to use in the other wireless network within a frame period.
Since the “Child Piconet” method for enabling a plurality of wireless networks to coexist with one another uses the parent-child network relationship on a time-sharing multiplexing basis, the child network must once be included in the parent network. This involves a network joining operation (hereinafter referred to as “association”), which makes the operation complicated.
If the child network cannot communicate with the control station for the parent network, the wireless networks cannot build the parent-child relationship.
According to the latter wireless-network coexisting method, the processing for allocating a band to use in the other wireless network in the frame period is necessary.
In other words, one wireless network must join the other wireless network, undergo a predetermined procedure, and then allocate the band to use in the other wireless network. Control thus becomes complicated.
In contrast, in the case of the UWB wireless communications network, data transmission/reception is performed by spreading the data over a very wide band. This makes impossible to employ a method for providing a plurality of channels in the frequency domain. In other words, a technique for multiplexing a network by using different frequency channels for corresponding wireless networks, as in the known wireless LAN, cannot be applied. It thus becomes difficult for a plurality of UWB wireless communications systems to coexist with one another in the same space.
Since the impulse signal sequence used by the UWB wireless communications scheme has no specific frequency carrier, carrier sense is difficult to perform. Therefore, for example, when the UWB wireless communications scheme is applied to the PHY layer of IEEE 802.15.3, access control using carrier sense standardized by the corresponding section cannot be performed since there is no specific carrier signal. The only possible choice is to use access control on a time-sharing multiplexing basis involving a plurality of channels in the time domain.